$.fn.grace_horse_rock = function() {
  var horse = this;

  if( ! horse.hasClass('rocking')) {
    var delay = 500; // delay between frames: 500 milliseconds
    horse.addClass('rocking');

    rock = function() {
      if(horse.data('hover')) {
        horse.toggleClass('left');
        // save timer ID so we can stop rocking
        horse.data('rockTimer', setTimeout(rock, delay));
      } else {
        horse.removeClass('rocking');
      }

    };
    rock();

  }

  return this;
};

$(function() {
  $('#masthead a').hover(function() {
    $(this).data('hover', true).grace_horse_rock();
  }, function() {
    $(this).data('hover', false);
  });
});